


Ache

by Tiara_of_Sapphires



Category: Fire Emblem: Fuukasetsugetsu | Fire Emblem: Three Houses
Genre: Breeding Kink, Explicit Sexual Content, F/M, Fire Emblem: Three Houses Church Route, Fire Emblem: Three Houses Spoilers, Knotting, Plot What Plot/Porn Without Plot, no beta we die like Glenn
Language: English
Status: Completed
Published: 2020-04-26
Updated: 2020-04-26
Packaged: 2021-03-01 19:00:33
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,083
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/23861950
Author URL: https://archiveofourown.org/users/Tiara_of_Sapphires/pseuds/Tiara_of_Sapphires
Summary: He needed her like he needed air. She was happy to oblige.
Relationships: My Unit | Byleth/Seteth
Comments: 14
Kudos: 250





	Ache

**Author's Note:**

> *flings this depraved thing at you and runs*  
> Enjoy!

It became really clear by midday that Seteth was avoiding her. The usual touches that she came to expect, a gentle brush of his hand on hers as he passed her in the monastery halls, the press of his hand on the small of her back, were nonexistent.

Instead, he seemed to make a point of keeping himself as far from her as possible. He even kept his office door closed and locked, keeping her from seeing him and pressing a kiss to his mouth as they continued through the day.

The last time she saw him that day, he picked up a small plate of food from the dining hall. She watched him, feeling like she was just blushing uncontrollably, as he glanced down at the food and walked out back the way he came. Now, that was definitely strange. He would have at least passed by where she sat to say that he was tired or retreating to his quarters to take his meal. To not even acknowledge her or glance in the direction where she sat was strange.

Her mouth twisted as the scraps left on her plate seemed suddenly very unappetizing. She needed to go after him, propriety be damned.

She frantically justified her reasoning in my mind. She was Archbishop and he her advisor. He was also...well, her lover? Fiancé?

He had called her ‘love’. He kissed her in the Goddess Tower. They made a promise to each other, though all that had come of it was more chaste kisses and sweet, soft-spoken words.

It felt almost scandalous as she navigated the halls towards his quarters. Neither of them was in a position to be open about their relationship, though it seemed like all of her former students and the rest of the monastery were at least vaguely aware that there was something deeper there.

As she got closer to his door, Byleth felt warm, the same way she felt when Seteth walked into the dining hall.

_Strange._

Her heart didn’t beat, but if it did, it would have been pounding when she stood in front of his door. It was really no different from the times she would approach him regarding Church business. And yet, there was something deep inside of her that knew that something was definitely different.

She knocked on the door, the same way she would if this were any other day.

“Seteth? It’s Byleth. Could you let me in?” she called out.

Hearing nothing, a trill of fear shot through her, emboldening her a little bit. Maybe he was so sick that he couldn't answer her. Maybe he had collapsed.

She knocked harder. “Seteth? Is something wrong?”

Silence, but she knew that he was in his quarters. There was nowhere else in the monastery where he would possibly be and there was no way that he had left the monastery grounds.

She knocked again, looking around to see if anyone was milling in the halls. To have the Archbishop slamming at her advisor’s door and yelling for him would only create questions.

Seeing no one, she pounded her fist against the door again. “Seteth, I will break down this door if you don’t open it in the next ten seconds.”

A moment of silence and she was seriously considering summoning whatever godly power she had to kick down the wooden door before she could hear some shuffling on the other side.

“Forgive me, I am not feeling well,” Seteth's voice sounded.

Byleth couldn’t help but pout that he wouldn’t open the door to at least tell her that. Even muffled, she could tell there was something off about his voice. She wasn’t sure what it was exactly, but it piqued her curiosity.

“Seteth, open the door. Maybe I can help,” she replied, coaxing.

A beat of silence.

“I could order you to open the door, as Archbishop and all that.”

There was a muffled sound from Seteth and finally the door opened to reveal Seteth. A blush lit up his cheeks and his pupils were blown wide, leaving only the faintest green ring. He had shed himself of his coat and stood in slacks and a fitted white shirt

“Byleth. What—what are you doing here?”

The use of her given name, no title, sent a pang through her. He only used her name in private, even then, it was a rare occasion. He backed away farther into his quarters and she followed, watching him.

“It doesn’t take much for me to see that something is wrong," Byleth mused.

She turned to shut the door behind her, locking it. It was for privacy’s sake, of course, but the simple act had her feeling strangely warm. Turning back, she watched as Seteth stood several feet away from her with his arms crossed tight over his chest.

“I am—I am ill.”

“Ill?” Byleth repeated.

She stepped closer to him and he held himself still, as if he was holding himself back from something. She reached up and pressed the back of her had to his forehead and down to his cheek. He was warm and she could even feel the faint sheen of sweat that covered his skin.

“It feels like you have a fever," she murmured. "Has it been like this all day?”

Instead of answering, immediately leaned into her touch. She turned her hand so her palm cupped his cheek and he moved to press a kiss to her skin. His teeth grazed her pulse point and she shivered.

A sound caught in the back of Seteth’s throat. His arms unfolded to hang by his sides, hands in tight fists.

Another pang of warmth shot through her, lower this time. This was different, no some kind of illness that would require a mage's help. This was...

“Seteth, what is this about?”

She moved her hand away, but immediately felt the loss of contact. Seteth jolted forward to follow her touch and reeled back when he realized himself.

“I—I apologize. This is very unprofessional of me.”

 _Unprofessional_. As if they weren’t engaged, as if they hadn’t found respite in each other’s embrace before. It had always been a simmer, never quite ‘going all the way’, as her students would call it.

“Seteth, what is going on? Tell me. I can’t help you if you don’t tell me.”

He backed away a step, but even then she could feel how warm he was. “I—it has been a long time," he started. "What happened to Rhea seems to have awoken something.”

 _What happened to Rhea._ That violent transformation and battle that nearly tore the monastery to the ground. That day when she learned Rhea, Seteth, and Flayn’s true natures.

Terror replaced the passion that had filled her moments earlier. It had taken months to repair Garreg Mach from the damage the final fight had caused. It had taken longer to even come close to making peace with what had happened, after Rhea’s burial.

“Are you going to transform like Rhea did?” she asked, voice shaking.

He shook his head and Byleth almost crumpled to the floor in relief. The idea of him buckling over in pain and turning into something unrecognizable and being forced to kill him was something that haunted her dreams more times that she could count.

“It has been months since that day. Why now?” Byleth whispered.

He bit his lips, eyes downcast. “I don’t know. Though I cannot turn into my winged form anymore, there are some things that—that remain.”

“Such as?”

He shuffled half a step closer to her and made a sound halfway between a grunt and a whine.

“Our breeding cycles. Though we did not adhere as strictly to them as lowly beasts do, it has been so long.”

“Breeding?” she repeated faintly.

Instinctively, Byleth glanced down to see the noticeable tent in his trousers. His hands were still in fists by his side, trembling in his incredible restraint.

“Oh,” she breathed. “So, you need—?”

She couldn’t even bring herself to finish the question that they both knew the answer to.

“I must admit, Byleth, that your presence here is very distracting.”

He had taken another step towards her, so they were almost as close as they were before. The blush was still in full-force and she could feel the heat radiating off of him.

Finding some courage, she reached up to touch his shoulder and Seteth immediately shuddered. His hands flew out to grab her hips and pulled her in, so that no space remained between their bodies. He ground his hips against her and he was so close to kissing her before he paused. His mouth hovered close to hers and she could feel his breath puff against her face.

He shook his head. “I apologize. I am imposing on you.”

Ever the gentleman. In another situation, she would’ve found it endearing. The warmth that seemed to consume her and her own arousal turned her knees to jelly and she was glad that Seteth had an iron-grip on her hips to keep her upright.

“I don’t recall saying that you were imposing on me,” Byleth said.

She skimmed the hand on his shoulder up to cup the back of his neck. The fine hairs on the back of his neck were tacky with sweat.

“We are promised to one another, are we not?” she asked.

Another shiver. “Yes, yes we are, but, joining with me now will not be the same as joining with a human.”

If it was meant to dissuade her, it didn’t work.

She pulled him closer and pressed her lips to his. For a brief instant, it was the same sort of kiss that they had shared in before, sweet and chaste.

It didn’t last long, as Seteth wrapped his arms around her and pressing her flush against him, kissing her deeply. He licked into her mouth, lacking all of his usual elegance. It was as if he wanted to consume her and she wanted to let him do just that.

They broke apart with an obscene sound, still breathing each other's air. He sounded as if he had sprinted across the monastery, out of breath.

“You—you want me?” Seteth breathed.

That should have been obvious. Byleth suppressed a laugh and instead responded with a kiss to his throat. He whined at the contact.

Whatever this was, be it a feat of magic or biology, it clearly brought him suffering. She supposed, if she were a crueler woman or if she had no emotional connection with him, she would have left him to relieve himself until the urge disappeared.

Now, that was something that brought a flash of heat that she wasn’t going to examine just yet.

“Will it bring you relief?” she asked.

Her mouth already felt swollen from kissing and she wanted more of it.

“Yes,” came the immediate answer, overeager.

Now, a shy blush from him and it would’ve been unbearably cute if she weren’t so turned on at the moment.

Byleth wrapped her arms around his neck.

“I want you to give it to me,” she breathed against his mouth. “ _Cichol_.”

That immediately set something off in him. He claimed her lips again with a growl, stripping her of her coat and leaving it in a puddle at her feet. Her shirt was next and she could hear the seams popping as he helped her pull it over her head.

He collected her in his arms and his hands stroked over her skin. She was already wet for him and all she wanted was for both of them to get naked.

“Couch?” she breathed.

“No. Bed. We—we will need the bed.”

She didn’t have the time to wonder what he meant by that before he leaned down and kissed her again. He led her by the hand through the little living area and into his bedroom.

As soon as the door closed behind him, Byleth made short work of Seteth's shirt. Now that she had the chance to touch him, she couldn’t resist. She stroked the pads of her fingers over his chest and admired the muscles that spoke of many hours training.

Seteth pawed at her remaining clothes, removing her bra and undoing the fastenings of her trousers before getting distracted by the swell of her breasts. He ducked down and sucked one nipple into his mouth. Byleth cried out and threaded her fingers through his hair to keep him pressed to her skin. He palmed at her other breast; the callouses from using a lance scraped over her skin and left chills in their wake.

They shed the rest of their clothes shortly afterwards, leaving nothing more than the touch of skin on skin.

Byleth couldn’t help but glance down at Seteth’s erection.

While he was hard and leaking, that wasn’t what caught her attention. He was swollen at the base of his cock, noticeably thicker than the rest of his shaft.

“What is that?”

She reached out to brush her fingers over his skin and up towards where his girth suddenly increased. She knew enough about anatomy to know that it wasn’t quite normal.

“A knot,” Seteth answered, catching her wrist to stop her ministrations. “I suppose not unlike some beasts. It will swell and lock me to you, keeping my release inside.”

He seemed strangely coherent and matter-of-fact for someone who was practically shaking with arousal. The crassness of his words belied the calm. She knew that Seteth, in his right mind, would never say anything close to that. He would blush and use the most veiled language.

He reached down; hands hovered just over her warm skin as if to ask for permission.

She grabbed his wrist and placed his hand over her cunt.

For a long moment, the world seemed to stand still. Then, he moved fingers back and pressed one into her pussy and the world started back again, only faster than before.

She clung to his shoulders and kissed him as prepared her. It was clumsy and messy and every time she brushed against his erection it looked as if he was about to collapse on the ground. She wasn’t doing much better, as fucked her with his fingers.

“How much longer?” she muttered, breaking off a particularly filthy kiss.

Seteth crooked his fingers, quickening his pace, driving to distraction. Pressing his face into her hair, he mumbled, “I don’t want to hurt you.”

She raised an eyebrow at that, as if he hadn’t seen her fight and defeat demonic beasts and whole battalions on the battlefield. She could handle him in bed.

After letting herself enjoy Seteth turning her knees into jelly for a few more moments, she caught his wrist and pulled him away. She whimpered a little at the loss of fullness, but quickly distracted herself by shoving him onto the bed. He hit the covers with a bounce and he watched her wide-eyed.

“Come on,” she breathed.

Seteth seemed to take the hint, crawling backwards to the edge of the bed as his erection bobbed between his legs. She followed, feeling oddly predatory as crawled after him. She straddled him and ground down on his length.

His hands flew to her hips.

“Byleth,” he hissed.

Now that she was looking down at him, it was easier for her to see the effect she had on him. Under the candlelight, his lips were swollen from kisses and his hair was mussed from where she threaded her fingers through. She could even see the point of his ear, revealed in another way how not human he was.

He really was handsome.

“Are—are you going to just keep staring?” he gasped. He bucked up impatiently against her.

She smiled indulgently and reached down to take his erection in hand. She pumped it a couple of times and felt his entire body shudder underneath her.

“Where are your manners?” Byleth muttered.

She wanted him inside her so bad but just couldn’t help but tease him. When his hold on her hips tightened, sure to make bruises, she grinned toothily.

“Please?”

She eased herself up onto her knees and rubbed the head of his cock against her folds. Lining the head with her entrance, she sunk down on him.

Byleth shivered with a sigh and Seteth groaned as she took him in as deep as she could. He was big, bigger than any partner she had before.

“You feel so good.” Seteth watched her with worshipful eyes as she lifted herself off until only the tip was inside before sinking back down.

Her breasts swung as she moved and Seteth caught them in his hands. He squeezed her breasts and flicked his fingers over her nipples until they were hardened peaks.

Byleth moaned and could only pick up the pace until she was riding him with fervor. It was almost as if she was the one who was in desperate need of release, not him.

It only took her hand stroking where their bodies met that the heat that had been simmering since she stepped into his quarters peaked. Seteth pulled her close as she shivered and tightened around him.

He didn’t loosen his grip. He braced his feet against the bed and started thrusting into her with short, fast movements.

Byleth gasped and mouthed at his chest as he took his own pleasure. She reached back and held herself open as the quick rhythm turned erratic. Seteth grunted and moaned as he got closer and closer to his release.

Something akin to a prayer escaped his lips before he slammed his hips into her one more time. He rocked her against him as he came inside of her. Byleth nuzzled against his throat as she felt some of his seed already dripping out of her. Distantly, she wondered how effective this knot of his was if his seed still leaked out of her.

Byleth braced her hands against his chest and pushed up. Seteth gasped with a flushed expression.

“That wasn’t so bad,” Byleth whispered, still out of breath.

Seteth shook his head, shivering underneath her.

“Not done yet.”

His hands tightened on her hips and pulled her down until he was pressed to the hilt inside of her. He was still hard and felt so big inside of her.

Not even after that, he still needed to come? What was this breeding cycle that intense? How long would she need to be in his bed for?

Not that she was complaining, as she still felt the aftershocks from her orgasm even now.

“Let me just—,” he muttered.

He flipped her over in a flurry of limbs, not slipping out an inch from her body.

“F—forgive me,” he gasped as soon as he was on top of her, between her legs. “I have been trying to hold myself back, but it is becoming increasingly difficult.”

She blinked at him in shock, already thinking about how sore she was going to be the next day. “That was you holding back?”

He stared at the comforter next to her shoulder.

“Every part of me wants to fill you up, so you’re—you’re full of my seed. It is primal and s—ah—sinful of me to want that.”

She watched his expression contort with guilt. His erection didn’t flag, so she was sure he didn’t feel _that_ guilty.

“I said yes, didn’t I? That I wanted you?”

His forehead thumped against her collarbone. “I—.”

She tightened her walls around him, punching a growl from his mouth. He grinded his hips almost harshly against her in retaliation. He found the juncture between her neck and shoulder and sucked a mark into it, sending shivers down her spine.

“This is what you want?” he hissed against her throat.

He sucked another mark there and soothed the sting with his tongue.

“Yes, Seteth,” she breathed. “You have me. You always have.”

She caught his face in her hands and kissed him slowly. She poured every ounce of her desire and her want for him in that one kiss and hoped that he would get message and stop denying himself.

Taking that as the permission it was, he bent her legs at the knee and bore down over her. Somehow, he was in deeper than he was before.

“Like that,” Byleth breathed.

Seteth thrust into her, fucking her into the bed with fervor.

Seteth had derided his mating cycle as beast-like. Byleth was beginning to see why. Even his irises seemed brighter where they weren’t subsumed by his pupils. His teeth were bared and his brow was tight in concentration.

He looked like a beast and she wanted to be consumed by him.

She held her legs open for him as he fucked her. She didn’t feel over-sensitive just yet. She chased her release as he pounded into her.

“I’m close.”

He fucked her, harder, faster. She could only grab at his arms and the comforter beneath them and hold on as her peak got closer and closer.

“I— _ah_ ,” he hissed. “Perfect. Perfect. I— _hah_ —Byleth. You’re perfect.”

He pumped his hips and she could feel him coming inside of her. It felt like he was coming forever, not stilling his hips as he emptied himself. Finally, he pressed hard into her, growling like a beast.

“Oh,” Byleth gasped.

She now knew what he had meant, how it was nothing like having sex with a human. The knot at the base of his cock swelled and got impossibly bigger inside of her, stretching her almost to the point of pain. She squirmed underneath him, whining softly.

Understanding turned her insides to liquid. This was the binding, beast-like. She was his as he was hers.

“You’re full of me,” he rasped against her throat.

His fingers skimmed over where their bodies met, finding her clit and rubbing in gentle circles. She gasped and tightened around him, pulling him impossibly deeper into her. All Byleth could do was mutter nonsense as Seteth brought her over the edge, a litany of pleas and his names spilling from her mouth.

Seteth ground his hips into her in response, as if there was anywhere deeper that he could go.

“Full of—our children?” Byleth gasped.

He had said that his love for her had no connection to their shared ancestry, though there was always the thought in her mind that they would be assisting in rebuilding a lost people.

He shivered in her arms and nuzzled at her throat.

“Yes.”

Now wasn’t the time to really think about the future in terms of children, but the thought of it brought a ballooning feeling in her chest.

He gently turned them so they faced each other. Byleth’s leg hitched over his hip, his cock still inside of her. Everything was too hot, bordering between overstimulation and pleasure.

“How—ah, how long before the swelling goes down?” Byleth asked, trying to catch her breath.

He pressed his forehead against her collarbone.

“I don't know. It could be a little bit.”

She hummed, stroking her hand over his chest and resting it over his still fast-beating heart. Being stuck with him, literally, wasn’t the worst thing in the world.

“We could always talk about the budget,” he suggested.

Byleth snorted as she pressed a kiss to her collarbone. “I think not. You wore me out.”

“Ah, I hope not.”

She looked up at him in confusion. Seteth stared at a space just over her head. A blush that spoke only of embarrassment and not arousal painted his face.

“Our—ah—breeding cycle takes a couple of days. It may be a few hours before the desire overtakes me again.”

“Days?” she repeated.

Seteth cleared his throat. “This isn’t a one-and-done situation, so to speak. I hope that won't be an issue.”

She shook her head, pushing away the sudden rush of imagination on how many times and in how many different ways they would be able to join together while Seteth was in this state. “Not at all, I just think we should somehow inform the staff that we will be indisposed for a few days.”

Glancing down between their bodies and how the knot had not let up its lock inside of her, she gave a wry smile.

“Tomorrow,” she murmured. “I’ll tell them tomorrow.”

“Tomorrow.”

She tucked her face against his chest. It was going to be an interesting next couple of days.

**Author's Note:**

> *blushes* Listen I needed to get some Setleth smut out of my system since they aren’t going to bang for a long time in my longfic.  
> Comments and kudos are greatly appreciated! I work in a clinic so days are long and difficult, even more so nowadays. Every kind word brings me more motivation to work on stuff like this!  
> [Here’s my Tumblr! Feel free to follow and drop Setleth drabble/fic requests!](https://tiaraofsapphires.tumblr.com/)  
> Cheers!


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